Chronic relapsing intestinal inflammation appears to be mediated by dysregulated innate and acquired immune responses to commensal (nonpathogenic) enteric bacteria in genetically predisposed individuals. We hypothesize that (1) various non-pathogenic commensal bacteria induce distinct disease phenotypes in genetically susceptible hosts due to either regionally restricted mucosal immune responses to bacterial antigens or b. selective epithelial adherence/ invasion in different regions of the intestine, and (2) a discrete number of E.faecalis antigens induce dysregulated T cell immune responses, leading to the induction and perpetuation of chronic intestinal inflammation. These hypotheses are addressed in the following Specific aims: 1. Identify the mechanisms by which E.faecalis and E. coli, alone and in combination, induce different disease phenotypes in the same genetically susceptible host, the IL-10-/- mouse, by: a. Comparing virulence factors (epithelial adherence and invasion) of E. coli and E.faecalis shown to differentially induce distinct disease phenotypes in monoassociated IL-10-/- mice to known adherent/ invasive E. coli (AIEC) strains isolated from human Crohn's disease patients, and randomly isolated commensal E.coli and E.faecalis strains from SPF IL-10-/- mice with colitis and WT mice. b. Determining in vivo regional differences in mucosal adherence and invasion by each bacterial species in IL-10-/- mice co-colonized with these two bacterial species. c. Investigating the ability of adoptively transferred CD4+ cells isolated from colitic IL-10-/'- mice monoassociated with E. coli or E.faecalis to induce regionally specific intestinal inflammation and variable disease kinetics in immunodeficient mice colonized with either or both bacterial species. 2. Identify dominant E.faecalis antigens activating T cells in monoassociated IL-10-/- mice by: a. Determining the dominant antigens (identified in an expression library screened by serologic responses) that induce interferon-gamma (IFN-gamma) responses in CD4+ T cells from the mesenteric lymph nodes of E.faecalis - monoassociated IL-10-/- mice with colitis. b. Identifying the specific epitope(s) involved in immune activation and subsequent perpetuation of intestinal inflammation in IL-10-/- mice through truncation mutagenesis. c. Determining if recombinant nonpathogenic bacteria engineered to expressing the dominant E.faecalis epitope can induce T cell - mediated colitis and duodenitis.